timepieces · 3 min read
The vote to retire the cesium second
The cesium atom has been keeping official time since 1967 — nine billion, one hundred ninety-two million, six hundred thirty-one thousand, seven hundred seventy oscillations per second, locked into the International System of Units at the 13th General Conference on Weights and Measures in Paris. On October 15, 2026, representatives of sixty-eight nations met at the Palais des Congrès de Versailles and voted to begin the work of replacing it.
The 28th General Conference on Weights and Measures (CGPM), convening October 13–15 under BIPM Director Annette Koo and CIPM President Dr. Wynand Louw, is the body that sets the SI. It meets every four years; its resolutions become the legal foundation of units used in commerce, science, and navigation worldwide. By October 2026, the pressure to act on the second had been building for a decade. The best optical atomic clocks — lasers interrogating single trapped ions or millions of laser-cooled atoms in a lattice — had surpassed the cesium standard by a factor of up to one hundred in accuracy. Several had reached a threshold that would have seemed absurd a decade earlier: they were more precise than the cesium definition itself. Their performance was no longer limited by their own physics. It was limited by what “second” officially meant.
Resolution 4, “On the future definition of the second,” formally acknowledged this and tasked the International Committee for Weights and Measures (CIPM) with bringing a concrete proposal for a new definition to the 29th CGPM in 2030. What that definition will look like remains unresolved. Two approaches were debated in Versailles: Option 1 is a single atomic transition — clean and simple, one species, one number, like the cesium hyperfine transition since 1967. Option 2 is an ensemble, a weighted geometric average of several optical transitions from different atomic species, offering redundancy at the cost of a more complicated definition. The resolution noted the standoff plainly: “A consensus has not been reached despite active scientific debate.”
The roadmap that preceded the vote — written by Noël Dimarcq of SYRTE Paris, President of the Consultative Committee for Time and Frequency, alongside thirty-five colleagues from NIST, PTB, INRIM, and other national metrology institutes — catalogued what still had to happen before 2030. Among the items: resolving “noticeable inconsistencies in some comparisons of optical frequency standards.” The world’s most accurate clocks, it turned out, do not always agree with each other. Some disagreements come from known and difficult-to-eliminate effects — blackbody radiation shifts, electric field perturbations, the gravitational potential of the lab floor. Others are still being traced. The formal vote to change the definition of time had to wait, in part, because the clocks competing to carry the new definition were still, occasionally, arguing about what time it was.
The situation has a precedent. The cesium second was defined in Paris in 1967, twelve years after Louis Essen measured the cesium hyperfine frequency at the National Physical Laboratory in Teddington. Resolution 4 of 2026 is the equivalent moment in that arc — not the final step, but the first formal commitment from the community of nations that the next definition is coming, and that the work to earn it is on a named schedule. For the first time in sixty years, the second has a planned successor.
The cesium clocks will keep running until 2030 — still the legal definition, still the reference against which every other clock is calibrated, still accurate to within one second over three hundred million years. In 2030, they will keep running. They just won’t define the second anymore.
Sources
- Resolution 4 (2026) — BIPM — primary source for the resolution text, the two options under consideration, the CIPM mandate, and the acknowledgment of inconsistencies between optical standards
- Roadmap towards the redefinition of the second — Dimarcq et al., Metrologia/NIST — technical criteria for redefinition, candidate species, single-species vs. ensemble debate, contributing authors
- 28th CGPM (2026) — BIPM — full resolutions list, meeting dates, location, and institutional context
- General Conference on Weights and Measures — Wikipedia — institutional history, 1967 SI second definition, meeting cadence